This invention relates to a hydraulic braking pressure control apparatus for vehicles, and more particularly to an apparatus having an input hydraulic chamber leading to an output port of a master cylinder, and an output hydraulic chamber communicating with a wheel brake and being adapted to generate a hydraulic braking pressure in accordance with a hydraulic pressure in the input hydraulic chamber, wherein the volume of the output hydraulic chamber can be increased in accordance with the supply of a hydraulic control pressure from an anti-lock control means to a control chamber when a wheel is about to be locked.
In a conventional hydraulic braking pressure control apparatus for vehicles of the kind described above, a piston is operated and moved in response to the introduction of hydraulic pressure into an input hydraulic chamber so as to reduce the volume of an output hydraulic chamber and thereby to generate a hydraulic braking pressure from the output hydraulic chamber in accordance with the pressure in the input hydraulic chamber. During anti-lock operation, the piston is displaced in a direction opposite to that in the above-mentioned case by means of a control liquid pressure supplied to a control chamber, thereby increasing the volume of the output hydraulic chamber.
In the conventional hydraulic braking pressure control apparatus, the hydraulic braking system is divided into two parts, one extending from a master cylinder to an input hydraulic chamber and the other extending from an output hydraulic chamber to a wheel brake. Therefore, when supplying working oil to the hydraulic system, it is necessary that working oil be supplied separately to these two parts.
Therefore, the assignee of the present invention has already proposed in U.S. application Ser. No. 755,502, filed July 16, 1985, now U.S. Pat. No. 4,627,670, dated Dec. 9, 1986. A hydraulic braking pressure control apparatus for vehicles, which unifies the hydraulic braking pressure system from the master cylinder to the wheel brake and makes it easy to supply working oil by disposing a first cylinder portion to which a first piston fits slidably, a second cylinder portion to which a second piston moving together with the first piston is fitted slidably, both of the first and second cylinder portions being disposed on the same axis while interposing a partition between them and a valve mechanism disposed in the partition between hydraulic input and output chambers and operative to be closed at the time of anti-lock control.
However, in this hydraulic braking pressure control apparatus described above, the valve closing operation of the valve mechanism is likely to be delayed at the start of the anti-lock control because the valve closing stroke of the valve body of the valve mechanism is relatively large. This problem can be solved by reducing the stroke of the valve body in the valve mechanism, but it is difficult to maintain accuracy when this is done.